The present invention is related to a magnetron employed in high frequency heating appliances such as microwave ovens and the like.
FIG. 11 indicates an example of a conventional magnetron 1 which is assembled in a microwave oven, or the like.
This magnetron 1 contains a cathode 3 whose central axis is directed along upper/lower directions, an anode tubular body 5 which coaxially encloses this cathode 3, an input-sided magnetic piece 7, a cathode-terminal conducting stem 31, an output-sided magnetic piece 13, a second metal cylinder 15, and a microwave radiating antenna 19. The input-sided magnetic piece 7 is provided at a lower opening end of the anode tubular body 5. The cathode-terminal conducting stem 31 is formed in such a way that this cathode-terminal conducting stem 31 is projected from a first metal cylinder 9 which covers this input-sided magnetic piece 7. The output-sided magnetic piece 13 is provided on an upper opening end of the anode tubular body 5. The second metal cylinder 15 covers this output-sided magnetic piece 13. The microwave radiating antenna 19 is formed on the second metal cylinder 15 in such a manner that this antenna 19 is projected via an insulating tube 17 made of ceramics from the second metal cylinder 15.
A plurality of anode vanes 20 are joined to an inner wall plane of the anode tubular body 5 in a radial shape, which are directed to a center axis of the anode tubular body 5. A strap-engaging concave portion 20a and a strap-inserting concave portion 20b are provided on an upper edge and a lower edge of each of these anode vanes 20 in such a manner that the position of the strap-engaging concave portion 20a is positionally shifted with respect to the position of the strap-inserting concave portion 20b along a radial direction, and both the strap-engaging concave portion 20a and the strap-inserting concave portion 20b are arranged in a reverse manner with respect to the upper edge and the lower edge. The strap-engaging concave portion 20a is employed so as to join a strap ring, whereas the strap-inserting concave portion 20b is employed so as to insert thereinto the strap ring in a non-contact manner.
Then, these anode vanes 20 arranged along a circumferential direction are electrically connected to each other every one vane, while any one of two strap rings 22 and 24 is joined to the strap-engaging concave portion 20a. These strap rings are a small-diameter strap ring 22 and a large-diameter strap ring 24, which are arranged on the center axis of the anode tubular body 5 in a coaxial manner.
One magnetic pole of a first ring-shaped permanent magnet 21 is magnetically coupled to the input-sided magnetic piece 7. This first ring-shaped permanent magnet 21 is made of ferrite, and is stacked on the outer edge plane of the input-sided magnetic piece 7 in a ring shape by which the first metal cylinder 9 is surrounded. Also, one magnetic pole of a second ring-shaped permanent magnet 23 is magnetically coupled to the output-sided magnetic piece 13. This second ring-shaped permanent magnet 23 is made of ferrite, and is stacked on the outer edge plane of the output-sided magnetic piece 13 in a ring shape by which the second metal cylinder 15 is surrounded.
A frame-shaped yoke 25 owns a through hole 25a which is used to insert the cathode-terminal conducting stem 31 into a lower edge portion thereof, while this frame-shaped yoke 25 is employed so as to magnetically couple the other magnetic pole of the first ring-shaped permanent magnet 21 to the other magnetic pole of the second ring-shaped permanent magnet 23.
Also, a large number of heat radiation fins 27 are mounted in a multiple stage on the outer peripheral plane of the anode tubular body 5. A metal filter case 29 is mounted on an outer surface of a lower edge portion of the frame-shaped yoke 25, while this metal filter 29 is employed in order to avoid such a condition that leaked electromagnetic waves are leaked out from the magnetron 1. The cathode-terminal conducting stem 31 having a smaller diameter than a diameter of the through hole 25a of the frame-shaped yoke 25 is tightly soldered to the first metal cylinder 9, while a cathode terminal 11a penetrates through an inner side of this cathode-terminal conducting stem 31, and then, is electrically connected to a lead wire 11.
A feed-through type capacitor 33 is mounted on a side surface portion of this filter case 29, whereas one end of a choke coil 35 is connected to the cathode terminal 11a of the cathode-terminal conducting stem 31 positioned within the filter case 29. The other end of this choke coil 35 is connected to a feed-through electrode of the capacitor 33 in order to constitute an LC filter circuit capable of preventing leaked electromagnetic waves.
In the conventional magnetron 1 constructed in the above-described manner, a choke ring 37 having a ¼-wavelength along the axial direction thereof is tightly soldered to the metal tube 15 in order to suppress high frequency noise which has been leaked on the side of the microwave radiating antenna 19.
On the other hand, as to magnetrons, there are regulations in order to prevent radiation noise (noise leakage) with respect to high frequency components, relatively-low frequency components of 30 to 1,000 MHz, and furthermore, base wave components (both bandwidths and sideband levels). In particular, there is a sever regulation with respect to the fifth harmonic wave.
The equipment of only the above-described choke ring 37 cannot sufficiently prevent radiation noise/leakages so as to clear such regulations for the radiation noise.
In general, when a spectrum of a base wave may become a clear waveform having a reduced sideband, an spectrum of an n-th wave (higher harmonic wave) also may become a clear waveform, so that radiation noise may be lowered. It should be understood that the generation of the sideband on the spectrum of the base wave is greatly influenced by a radius “Rp” of a central flat portion of the output-sided magnetic piece 13.
With respect to the flat portion of the output-sided magnetic piece 13, changes in the spectra of the base wave are represented in FIG. 12(a) to FIG. 12(e) when the radius “Rp” of this flat portion is gradually increased in a flat region in the proximity to each of the anode vanes 20 in order to concentrate magnetic flux into the effective space within the anode tubular body 5.
In FIG. 12(a) to FIG. 12(e), when a radial dimension of an outer circumference of the small-diameter strap ring 22 was “Rs1” and a radial dimension “Rs2” of an inner circumference of the large-diameter strap ring 24, while these radial dimensions “Rs1” and “Rs2” were employed as a reference radius, base wave spectra was measured by increasing/decreasing the radius “Rp” of the above-explained flat portion.
FIG. 12(a) shows a base wave spectrum when Rp<Rs1; FIG. 12(b) indicates a base wave spectrum when Rp=Rs1; FIG. 12(c) shows a base wave spectrum when Rp=(Rs1+Rs2)/2; FIG. 12(d) indicates a base wave spectrum when Rp=Rs2; and FIG. 12(e) shows a base wave spectrum when Rp<Rs2.
As apparent from the respective diagrams, such a trend is represented. That is, when the radius “Rp” of the flat portion of the output-sided magnetic piece 13 is increased (namely, difference with respect to choke diameter is widened), the generations of the sidebands are reduced in response to this increased radius, and thus, the resulting spectra may become clear.
In an actual case, when a noise level in the vicinity of 2.4 GHz is measured, as indicated in FIG. 13, the noise level is rapidly attenuated if the radius “Rp” of the flat portion exceeds the radial dimension “Rs1” of the small-diameter strap ring 22.
Accordingly, generally speaking, considering such a trend, the conventional magnetrons have been manufactured so as to capable of preventing the radiation noise/leakages, since the radius “Rp” of the flat portion of the output-sided magnetic piece 13 is made larger than the radial dimension of the large-diameter strap ring 24.
However, when the radius “Rp” of the flat portion of the output-sided magnetic piece 13 is made larger than the radial dimension of the large-diameter strap ring 24, although the reduction of the radiation noise can be realized, there is such a problem that, as may be understood from the base wave spectrum level of FIG. 12(e), the oscillation efficiency is lowered.
Very recently, a specific attention has been paid to noise in the 2.2 GHz range (band) among the radiation noise. There is such a trend that this noise of the 2.2 GHz range easily may be produced when the oscillation efficiency is increased. FIG. 10 shows a noise waveform of the 2.4 GHz range, and also, a noise waveform of the 2.2 GHz range. In this drawing, a right portion corresponds to the noise in the 2.4 GHz range and a left portion corresponds to the noise in the 2.2 GHz range, as viewed in the drawing.